1. Field of the Invention
This invention relates to the field of optical filters based on Fabry-Pérot cavities and characterized by a large filtering surface. It is used particularly in the field of optical filter arrays, and in particular RGB filters for use in visible imaging systems.
2. Description of Related Art
Visible imaging systems commonly comprise an array of photosites, CCD or CMOS for example, sensitive in a wide range of visible radiation, and, in particular, sensitive at the same time to red, blue and green wavelengths. As is known per se, each photosite is then specialized in the detection of one of these wavelengths by superposing an array of red, green and blue filters distributed in a Bayer array.
The best known filter arrays are composed of colored resins deposited on the photosite array. Colored resin-based filter arrays of this type are however sensitive to the angle of incidence of the light, a wide angle of incidence bringing about a more substantial propagation length in the resin.
Furthermore, with colored resins, it is difficult to access very small pixel sizes, i.e. a size of less than one micrometer, on the one hand because it may be difficult to reduce the thickness of the resins owing to their limited pigment density, which causes cross-talk between the pixels, and on the other hand because of the transition zones between adjacent pixels which are of not inconsiderable dimensions, because of the not fully accurate successive deposition of the resins associated with the different colors (the side slope is not preserved as the depositions are made).
Furthermore, as is known per se, visible imaging systems are commonly provided with an infrared filter placed in front of the photosite array. Indeed, CCD technology photosites, and even more those based on CMOS technology, have very high sensitivity in the infrared, which is detrimental to the quality of the detection in visible light if no measures are taken.
In fact, an infrared filter is sensitive to the angle of incidence of the radiation. In particular, its filter cut-off shifts as the angle of incidence increases (i.e. moves away from the normal incidence), undergoing a shift towards blue. Thus, for wide angles of incidence, a useful part of the visible spectrum is cut off, particularly in the red. It is therefore the infrared resin-filter association which is quite sensitive to the angle of incidence.
Filter arrays based on Fabry-Pérot cavities have been designed to provide colored and infrared filtering simultaneously. These arrays conventionally include a plurality of metal layers separated by one or more dielectric layers, the refractive index and thickness thereof being selected to set the wavelength of the Fabry-Pérot cavities. The dielectric layers are conventionally composed of a single material so that the refractive index thereof is constant over the whole array. Thus, to obtain Fabry-Pérot cavities set to different wavelengths, but juxtaposed relative to each other, in a Bayer array for example, an array is then produced with one or more dielectric layers of variable thickness, the thickness being used to set the wavelength of the cavities. Reference may be made for example to the document U.S. Pat. No. 6,031,653 for further details about arrays of Fabry-Pérot cavities of this type. With these metal-dielectric filters, the infrared filter is no longer necessary since the infrared is cut by the metal.
This solution addresses the problem of filter thickness which is reduced by roughly a factor of 2, but does not significantly improve the dependence of the spectral responses with the angle of incidence, nor the problem of the transition zones between pixels inherent in the non-planarity of the filters.
Manufacturing an array of Fabry-Pérot cavities of variable thickness is moreover quite restricting in terms of industrial process since it requires a great many masking and engraving steps which considerably extend the manufacturing time and reduce the final cost gain of the array achieved by eliminating the infrared filter. Furthermore, array planarization is generally required since its surface is subsequently used to form micro-lenses. An additional thickness is therefore added to the stack of Fabry-Pérot cavities, the effect of which is to increase the cross-talk effect which is directly related to the thickness of the materials passed through to reach the photosites, even if it remains below the levels encountered with resin filters.
Document EP 1 592 067 proposes, in the embodiment shown in FIGS. 13A to 13E, an array of Fabry-Pérot cavities of constant thickness. Rather than using the thickness of the dielectric layer or layers to set the wavelength of the cavities, this document proposes using their refractive index.
FIG. 1, which re-uses FIG. 13D in this document, is a cross-section view of an array 10 of Fabry-Pérot cavities, showing three juxtaposed Fabry-Pérot cavities forming a blue transmission filter 12, a red transmission filter 14 and a green transmission filter 16 respectively.
The array 10 comprises:                an insulating substrate 20;        a first alternation 22 of dielectric layers of SiO2 and TiO2, deposited on the substrate 20 and forming a semi-reflective surface;        a dielectric layer 24 of variable refractive index        a second alternation 26 of dielectric layers of SiO2 and TiO2, deposited on the dielectric layer 24 and forming a semi-reflective surface.        
The dielectric layer 24 is formed of three distinct zones of different average refractive index, namely a first zone 28 composed of TiO2, a second zone 30 composed of SiO2, and a third zone 32 formed between the first and second zones 28, 30 and composed of a periodic network of bands of TiO2 implemented in a layer of SiO2. The TiO2 represents ⅕ of the volume of the third zone 32, and therefore the SiO2 represents ⅘ of this volume, so that the average refractive index of the zone 32 is equal to
                              1          5                ×                  n                      TiO            2                    2                    +                        4          5                ×                  n                      SiO            2                    2                      .
This structure thus forms three juxtaposed Fabry-Pérot cavities, set to a blue, red and green wavelength respectively.
However, the wavelength at which a resonance is obtained in a Fabry-Pérot cavity, and therefore the desired effect, namely a narrow transmission bandwidth around the wavelength, depends not only on the refractive index and on the thickness of the dielectric layer, but also on the angle of incidence of the electromagnetic radiation on the cavity.
In fact, cases of radiation with a constant angle of incidence on the entire surface of the array of Fabry-Pérot cavities are very rare.
In particular, in the field of imaging, the photosite array on which the array of cavities is superposed is always placed in the focal plane of an optic in order to form an image of the scene observed on the sensor. This type of detector is shown in FIG. 2. FIG. 2 shows diagrammatically a detector 40, such as a camera for example, comprising an optic 42, of optical axis OX, and a plane sensor 44. The sensor 44 includes a detector circuit 46 that has a photosite array placed in the focal plane of the optic 42, and an array 48 of Fabry-Pérot cavities superposed on the detector circuit 46 and similar to the one in document EP 1 592 067.
As is known per se, the optic 42 forms a substantially spherical image of the scene. A spherical wave front 50 is here shown out-sized in FIG. 2 to illustrate the problem of the variation in the angle of incidence of the radiation incident on the array 48. It is thus remarkable that this radiation has a normal incidence on the array 48 on the optical axis OX and a non-normal incidence elsewhere. So for example, it is not uncommon in respect of the sizes of prior art sensors 44 that the angle of incidence θL at the edge of the array 48 is equal to 20°, the angle of incidence being identified relative to the norm at the plane of the array 48.
FIG. 3 shows the influence of the angle of incidence on the position in the spectrum of the transmission bandwidth of a Fabry-Pérot cavity. The “C” transmission response is for example that of the cavity 52 placed on the optical axis OX and the “B” transmission response is for example that of the cavity 54 placed at the edge of the array 48, shown in FIG. 2, the two cavities 52 and 54 being identical and set to a green wavelength. The cavity 52 on the optical axis OX receives a radiation of normal incidence whereas the cavity at the edge of the array 48 receives a radiation of angle of incidence equal to 20°.
As may be noted, there is a substantial shift between these two transmission responses, the “B” response being furthermore close to blue whereas the C response corresponds to a green wavelength. There is therefore a great variability in the selected wavelengths depending on the position of the cavities in the array 48. The image detected by the detector circuit 46 is not therefore faithful to the actual colors of the scene observed.